The present invention relates to a method of producing sodium dithionite, commonly known as sodium hydrosulfite. More particularly, this invention relates to the preparation of anhydrous sodium dithionite by the reaction of sulfur dioxide and sodium amalgam in formamide solvent.
Sodium dithionite is commonly used as a reducing agent with principal uses in the vat dying of textiles and in the bleaching of wood pulp. Various processes have been developed for producing sodium dithionite either in the form of a solution or as an anhydrous salt. These include a formate process illustrated by U.S. Pat. No. 3,576,598, a zinc process illustrated by U.S. Pat. No. 2,226,576, aqueous amalgam processes illustrated by U.S. Pat. Nos. 2,938,771 and 4,100,098, and non-aqueous amalgam processes illustrated by British Pat. No. 786,212 and Rinker et al., I and E C Prod. Res. and Dev., Vol. 8, pp. 338-347 (1969).
Production of sodium dithionite by either aqueous or non-aqueous amalgam processes has often suffered from mercury contamination of the solid product. This has resulted from dispersion of mercury and/or amalgam particles in the aqueous or non-aqueous media and subsequent contamination of the solid product.
Rinker et al. describe the production of sodium dithionite by contacting sulfur dioxide in a non-aqueous solvent with sodium amalgam. The non-aqueous solvent is selected from among dimethylformamide, dimethyl sulfoxide and formamide. Formamide is found to eliminate dispersion of the sodium amalgam. Sodium dithionite prepared in formamide may be precipitated by the addition of dimethylformamide or acetone to the sodium dithionite-formamide solution. While yields of sodium dithionite on the order of 90 percent and 80 percent may be expected using dimethylformamide and dimethyl sulfoxide respectively, the yield of sodium dithionite using formamide as the solvent is usually less than 65 percent.
Solid, anhydrous sodium dithionite is relatively more stable than either the hydrated salt or an aqueous sodium dithionite solution. Aqueous sodium dithionite solutions are decomposed by acidic conditions. Such decomposition and the cost of transporting the aqueous solutions have dictated that sodium dithionite solutions be produced in relative proximity to the ultimate consumer. There is, therefore, a need for a stable solid sodium dithionite which would facilitate a wider geographical distribution of sodium dithionite.